The objective of the proposed work is to determine the mechanism of particle maturation for the HK97 virus, a highly accessible model system. Virtually all complex viruses initially assemble into non-infectious, often fragile, particles that are called provirions. Encoded in their structure is a program that directs particle conformational changes leading to a robust, infectious virion. Six different intermediates of this maturation have been isolated for HK97. Previous support for this work lead to the near-atomic resolution structures of three intermediates and the discovery that early maturation is an exothermic process and later maturation is driven by a Brownian ratchet mechanism. We now propose to determine the structures of three more intermediates and to study the transitions between all of the intermediates with single particle and ensemble methods. Our goal is to understand how the expression of two viral gene products creates a meta-stable particle that releases ~20kCal/mole of energy in the initial step of maturation and then proceeds by biasing thermal fluctuations with a covalent cross-link to complete the maturation process. We are also investigating the use of the mature particles as a potential delivery system for drugs in nano medicine.